The rest of the paper is organized as follows: we first describe the solvation structure of Na + and Cl −. related to the relative stability of the two metastable states, with first principles calculations allowing us to examine the effect of water self-dissociation on ion-water and ion-ion interactions. We also find quantitative differences between results obtained with first principles simulations and empirical force fields, e.g. We find key changes of ion-water and ion-ion interactions occurring at extreme conditions, which affect the free energy surface (FES) of the salt in water, in particular the relative stability of contact and separated ion pair configurations, the barrier between them and the minimum free energy path (MFEP) connecting the two metastable states. conditions similar to those at the bottom of the Earth’s upper mantle 31, 32. We carry out free energy calculations using first principles molecular dynamics (FPMD) simulations with the PBE exchange-correlation functional and empirical molecular dynamics (MD) simulations, coupled with enhanced sampling methods. Here, we report a computational study of ion pairing in NaCl dissolved in water at conditions relevant to the Earthʼs mantle, in the dilute limit, corresponding to a concentration similar to that of the average ocean salinity. No such study of ion pairing has yet been reported at high P and T, due to the many requirements in simulating solutions at extreme conditions, including a proper description of water dissociation and of the substantial changes in hydrogen bonding relative to ambient conditions 29, 30, 31. However experiments are not available at higher pressure.Ī number of simulations have been performed at ambient conditions to study the kinetics and thermodynamics of the CIP/SIP conversion 23, 24, 25, 26, 27, 28, with focus on NaCl. Usually salt solutions are studied by Raman spectroscopy 21, 22 and data have so far been reported up to ~700 K and ~1.5 GPa, showing that, similar to ambient conditions, there are two stable ion-pair configurations present in simple salt solutions, such as NaCl: the contact ion-pair (CIP) and the solvent-shared ion-pair (SIP), where the ions are separated by at least one water solvation shell. At high temperature and pressure, where water becomes highly corrosive, only few probes are available. In particular, the association of oppositely charged ions (ion-pairing) deserves special attention since it is the first step in the nucleation process of salts, and ultimately affects the conductivity of saline solutions 14, 15, 16.Īt ambient conditions, ion pairing in water has been investigated by several experimental techniques, including X-ray scattering and X-ray absorption 17, neutron diffraction isotope substitution 18, and dielectric relaxation spectroscopy 19, 20. Despite the key role of salts in water in determining the properties of the Earthʼs mantle constituents, very little is known about the relation between the macroscopic physical properties of saline solutions at high P − T and their molecular structure. ![]() For example, at high pressure ( P ~ 1 GPa) and temperature ( T ~ 1000 K) the presence of Na + and Cl − dissolved in water is known to greatly increase the electrical conductivity of aqueous fluids 9, 10, 11 while substantially decreasing the activity of water 12, 13 and increasing the solubility of several crustal rocks 6, 12. These fluids are saline, with properties dependent on the ionic concentration 6, 7, 8. In the Earth’s mantle water-rich fluids play a key role in many processes, including metasomatism 1, 2, magma production in subduction zones 3 and the Earth’s carbon cycle 4, 5. The minimum free energy path between the CIP and SIP becomes smoother at high pressure, and the relative stability of the two configurations is affected by water self-dissociation, which can only be described properly by FP simulations. We find that the free energy barrier between the CIP and SIP minima increases at extreme conditions, and that the stability of the CIP is enhanced in FP simulations, consistent with the decrease of the dielectric constant of water. Similar to ambient conditions, we observe two metastable states of the salt: the contact (CIP) and the solvent-shared ion-pair (SIP), which are entropically and enthalpically favored, respectively. ![]() We report first principles (FP) and classical molecular dynamics simulations of NaCl in the dilute limit, at temperatures and pressures relevant to the Earth’s upper mantle. The investigation of salts in water at extreme conditions is crucial to understanding the properties of aqueous fluids in the Earth.
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